Fiber-reinforced composite materials composed of reinforcing fibers such as glass fibers, carbon fibers, and aramid fibers and cured products of thermosetting resins such as unsaturated polyester resins, vinyl ester resins, epoxy resins, phenol resins, cyanate ester resins, and bismaleimide resins are lightweight and have excellent mechanical properties such as strength and elastic modulus, and are therefore used in various fields such as aircraft structural parts, spacecraft structural parts, vehicle structural parts, ship structural parts, civil engineering and construction materials, and sporting goods. Particularly, fiber-reinforced composite materials using continuous fibers are used for applications requiring high performance. In this case, carbon fibers excellent in specific strength and specific elastic modulus are often used as reinforcing fibers and epoxy resins excellent in adhesion to carbon fibers are often used as matrix resins.
However, epoxy resin cured products obtained by curing epoxy resin compositions are generally brittle, and therefore an improvement in the toughness of epoxy resin cured products is an important issue in improving the impact resistance and fatigue resistance of fiber-reinforced composite materials. In addition, it is also necessary to suppress crack propagation from holes to improve the open-hole tensile strength of fiber-reinforced composite materials. Also from such a viewpoint, an improvement in the toughness of epoxy resin cured products is an important issue.
It is known that the toughness of a cured product of an epoxy resin composition can be improved by adding rubber or a thermoplastic polymer to the epoxy resin composition. As a method for adding rubber, a method using rubber such as carboxyl-terminated butadiene-acrylonitrile copolymer (CTBN) rubber or nitrile rubber has been proposed (see, for example, Patent Documents 1 and 2).
According to this method, rubber is once dissolved in an epoxy resin composition, but thereafter phase separation occurs during curing process. This causes a problem that a desired toughness-improving effect cannot be obtained due to a change in the morphology of a cured product depending on the kind of epoxy resin composition used or curing conditions. In addition, a rubber component is partially dissolved in an epoxy resin phase of an epoxy resin cured product, which causes a problem that the viscosity of the epoxy resin composition is increased and the Tg and elastic modulus of the epoxy resin cured product are lowered.
In order to solve such problems, various methods using polymer particles substantially insoluble in epoxy resins have been proposed. For example, a method using core-shell polymer particles formed by partially or fully coating the surface of particles as a core mainly made of a polymer with a polymer different from the polymer constituting the core by, for example, graft polymerization has been proposed (see, for example, Patent Documents 3 to 6). It is known that such a method makes it possible to suppress an increase in the viscosity of an epoxy resin composition and a lowering of the Tg of an epoxy resin cured product.
However, in order to obtain a sufficient toughness-improving effect, it is necessary to add a large amount of core-shell polymer particles. Therefore, there is a remaining problem that addition of a large amount of core-shell polymer particles lowers the elastic modulus of an epoxy resin cured product, thereby lowering the compressive strength of a fiber-reinforced composite material in its fiber direction.    Patent Document 1: JP-B-61-29613    Patent Document 2: JP-B-62-34251    Patent Document 3: JP-A-5-65391    Patent Document 4: JP-A-7-224144    Patent Document 5: JP-A-2003-277579    Patent Document 6: JP-A-2006-257391